Such a test method with an ultrasonic wave propagated through the air as shown in FIG. 8 is known where the ultrasonic wave is propagated from a probe to a test piece which in turn generates a plate wave and allows the probe to measure the plate wave generated in the test piece (See Patent Document 1).
Another test method is also disclosed in Non-patent Document 1 for improving the intensity of transmission and reception of incident/leak signals with the use of a probe which is curved along the horizontal direction.                Patent Document 1: Japanese Patent Laid-open Publication No. 2005-055197.        Non-Patent Document 1: “Air-coupled ultrasonic inspection method for CFRP (2nd report)” by Shigeyuki Matsubara et al, The Japanese Society for Non-Destructive Inspection, Autumn Proceedings in Heisei 15, pp. 3-4.        
In common, the probe employed in a test of this type is arranged for transmitting and receiving an ultrasonic wave along one direction. Accordingly, during the propagation of the ultrasonic wave from the probe, the ultrasonic wave propagating from the probe falls at a uniform degree of the incident angle θ on the test piece.
And now, FIGS. 8 and 9 illustrate the relationship between the product FT (kHz·m) of the frequency of the plate wave and the thickness of the test piece and the incident angle θ(°) at which the ultrasonic wave is directed to generate a plate wave, at different modes of the plate wave denoted by A0 to A5 and S0 to S7. Accordingly, for example, to match the product FT3 at the mode S1 where the frequency of the plate wave is F and the thickness of the test piece is T3, the incident angle of θ3 is desired and, in practice, set up with the use of a proper jig.
However, when the thickness at the incident point is changed from T=T3 which has been predicted to T=T3+dT=T4, the incident angle turns up to θ=θ4 which disallows the ultrasonic wave to penetrate the test piece for generating a plate wave. Also, when the incident angle θ is relatively changed by the effect of undulations of the surface of the test piece, it disallows the ultrasonic wave to penetrate the test piece for generating a plate wave with the same reason. The relationship between the two factors depends largely on the material properties of the test piece. Accordingly, the probe has to be reset every time when the test piece is changed in the thickness or the material properties. Moreover, any change in the condition of the material properties may inhibit the ultrasonic testing action itself from being conducted.